arm64: dts: imx8m: disable smart eee for FEC PHY

As commit 390b4cad ("net: phy: at803x: add support for configuring SmartEEE")
described, disable PHY smart eee by default.

Signed-off-by: Joakim Zhang <qiangqing.zhang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>